1. Field of the Invention
The present invention relates to a method for measuring beam spot luminescence distribution and a device therefor to measure luminescence distribution within a beam spot of an electron beam irradiated on a tube surface of a color cathode ray tube (hereinafter referred to simply as a "CRT").
2. Description of the Prior Art
FIG. 1 is a perspective view of the inside of a CRT according to a measuring method for beam spot luminescence distribution in the prior art. In FIG. 1, numeral 1 designates a tube surface serving as a display surface of the CRT. Numerals 2, 2a represent fluorescent substances for emitting red, green, and blue (R, G and B light) as picture elements applied to the tube surface 1. The hatched fluorescent substance 2a indicates picture element under luminescence. A shadow mask 3 is spaced a prescribed distance from the tube surface 1. Beam passing holes 4, 4a correspond to the fluorescent substances 2, 2a respectively. An electron beam is emitted from an electron gun (not shown). A beam spot 5a of the electron beam 5 is formed on the shadow mask 3 and a virtual beam spot 5b is formed responsive to the beam spot 5a.
An electron beam 5 emitted from an electron gun passes through a plurality of (four in the figure) holes 4a of a shadow mask 3 and irradiates a plurality of fluorescent substances 2a corresponding to the holes 4a causing the fluorescent substances 2a to luminesce. Accordingly, the brightness of each of these fluorescent substances 2a is measured and the luminescence distribution within one beam spot 5b can be determined.
FIG. 2 is a brightness distribution characteristic diagram of each fluorescent substance 2a when eight fluorescent substances 2a luminesce within the beam spot 5b. 2b correspond to respective fluorescent substances 2a. Data 6 composed of three discrete values derived from the characteristic diagram is compared with standard data shown in FIG. 3(A). If the shape of the data 6 is close to the shape shown in FIG. 3(B), the brightness is determined to be low, and if the shape of the data 6 is close to the shape shown in FIG. 3(C), the focus is determined to be low.
Since the measuring for beam spot luminescence distribution is carried out as mentioned above, the electron beam 5 is intercepted by the shadow mask 3, and only the beam passing through the holes 4a makes the fluorescent substances 2a luminesce as clearly seen from FIG. 1. Consequently, the luminescence distribution within the beam spot 5b is determined only by the sampling data correspond to the fluorescent substances 2a which luminesce. The brightness of a part associated with the electron beam 5 being intercepted by the shadow mask 3 cannot be determined. Therefore, in the case of a high resolution CRT with fine beam diameters, the number of data points able to be sampled becomes 3-4. Therefore highly accurate measurement cannot be carried out because the beam spot 5b is very small.